Determine differential ratio w/o counting wheel/shaft turns

[Cupholder Zealot]

I have two project E28s right now, and I am pretty sure the junker has a shorter ratio in the rear end. Any way to tell the diff ratio without jacking up a wheel and counting the number of turns of the wheel/driveshaft?

Thanks

[Kyle in NO]

Yeah, read the tag affixed to the diff cover.

[davintosh]

What Kyle said. They look like this...



This one is a medium-case diff from an e32*, but the tags on the small diffs are similar.

* this one also happens to be for sale.
/shameless_plug

[Cupholder Zealot]

Wow thats unbelievable! Can't wait to look. Last time I was changing gear ratios was a Ford 9" rear end, back in highschool. Even if there was a tag, you'd never assume it was right.

So I take it people don't change out the "ring and pinion" in these axles?

Thanks for the reply!

[Shawn D.]

So I take it people don't change out the "ring and pinion" in these axles?

Almost never. Why would they? AFAIK, there aren't any aftermarket gearsets, the OE stuff is expensive to buy new, and the gearset is very durable.

[davintosh]

So I take it people don't change out the "ring and pinion" in these axles?

Almost never. Why would they? AFAIK, there aren't any aftermarket gearsets, the OE stuff is expensive to buy new, and the gearset is very durable.

Plus, if you wanted to change gearing, buying a used diff and swapping them out probably isn't much more difficult or expensive than changing gearsets.

[1st 5er]

What Kyle said. They look like this...



This one is a medium-case diff from an e32*, but the tags on the small diffs are similar.

* this one also happens to be for sale.
/shameless_plug

PM incoming.

[justin517]

So I take it people don't change out the "ring and pinion" in these axles?

Almost never. Why would they? AFAIK, there aren't any aftermarket gearsets, the OE stuff is expensive to buy new, and the gearset is very durable.

Plus, if you wanted to change gearing, buying a used diff and swapping them out probably isn't much more difficult or expensive than changing gearsets.

swapping the whole diff is actually easier than setting up a new gearset, just R+R the whole diff rather than having to mess with shims and such to get the gears meshed properly.

[86GT635]

davin, do you know what the stock ratio is for the e32 5 speeds? I've got a lsd, but no idea on ratio. tag is no good.

[mooseheadm5]

davin, do you know what the stock ratio is for the e32 5 speeds? I've got a lsd, but no idea on ratio. tag is no good.

3.64

[Cupholder Zealot]

I finally looked under the two cars, and had a nice surprise. The driver has 3.25, and the junker has 3.46. Does the "S" mean anything?

Thanks again

[Shawn D.]

Does the "S" mean anything?

It means it's limited-slip.

[Cupholder Zealot]

Ok, I guess I totally lucked out then...but the LSD is in the 3.25. So I guess I'll be pulling out some shims and doing a gearset swap to get a 3.46 LSD.

Thanks for the help on this!!

[1st 5er]

So I guess I'll be pulling out some shims and doing a gearset swap to get a 3.46 LSD.

From what I've read, it ain't that simple.

[mooseheadm5]

Ok, I guess I totally lucked out then...but the LSD is in the 3.25. So I guess I'll be pulling out some shims and doing a gearset swap to get a 3.46 LSD.

Thanks for the help on this!!

I know for a fact it isn't that simple. You really have to know what you are doing to make that happen. It isn't like building a domestic rear at all.

[Cupholder Zealot]

Hmm, I don't have my Bentley manual in front of me...will that give me a rough idea of what is involved, in order to determine if its worth a try?

I have built manual transmissions, if thats any consolation.

Although I don't mind one wheeled burners, I did plan on a turbo setup, so the LSD would be a nice addition.

I guess worst case, I will pull apart the 3.25 and see what it involves before getting into the 3.46. Any more suggestions on the specifics of the innards involved would be great...now I'm too curious to have to wait until the end of the work day to jump into my manual.

[mooseheadm5]

There is no build info in the Bentley for a diff. I don't believe that BMW sells individual parts any more either. If you plan for a turbo, you want the 3.25 anyway so your turbo will spool faster. I would pull the 3.46 apart before the 3.25. What it takes is a crush sleeve for the pinion. Once you take it apart, you can't reuse that.

[1st 5er]

This is even beyond Bentley skillz.
Sell the 3.25 and buy a 3.46, or that ^^^.